Screw transmission, nut cable attached to screw

ABSTRACT

A cable or another longilinear transmission component ( 4 ) is set ( 3 ) into the centre of an axial cavity ( 2 ) of a screw ( 1 ) belonging to a screw and nut movement conversion unit. By means of this arrangement, bending moments exerted on the screw between the point of attachment of the component ( 4 ) and the nut have only a small value, which guarantees the screw ( 1 ) against bending and sticking and the whole mechanism against excessive friction.

[0001] The subject of the invention is a transmission including a screw,a nut, and a longilinear component such as a cable attached to thescrew, and the movement of which is controlled by means of the screw, bya conversion of movement between rotation of the nut and translation ofthe screw.

[0002] There are many ways of driving motion transmission cables,whether or not wound in a loop but wherein the motor speed should almostalways be geared down to increase the force transmitted by the cable.One system encountered includes two pulleys of different diameters onwhich the cable is wound, possibly making several revolutions around thepulley of smaller diameter in order to increase grip. The small pulleyis the driver and communicates a smaller rotation but a more substantialtorque to the large pulley. A drawback of this system is that it has alarge space requirement, the two pulleys having significant minimumdiameters, that slipping is difficult to eliminate completely and thatthe cable portion on the small pulley is subject to a significantangular speed which increases its fatigue and may therefore cause it towear quickly.

[0003] Another system includes gear reductions, but these mechanicalmeans include substantial play which makes them inaccurate and theirspace requirement is still significant. In other designs, cablemovements are controlled by means of a block gearing down the motionbetween a drive cable and a driven cable, but blocks are complex piecesof equipment and the reliability of the whole is not guaranteed.

[0004] One final group of motion to cable transmissions, to which theinvention is related, includes screw and nut systems one of the elementsof which is driven by the motor and the other is attached to the cable.These systems achieve excellent speed reduction and good nominalresistance, but a significant problem emerges in that the componentshave to be guided and that possible alignment defects have to be takeninto account between the screw, the nut and the cable.

[0005] This latter phenomenon causes significant friction between thescrew and the nut and bending moments on the screw, which compromise theyield and lifespan that might have been expected. Mechanisms responsiblefor guiding the screw and the nut while keeping them in alignment, andwhich might have overcome these defects, are too cumbersome or increasefriction still further. It is for this reason that the proposal has beenmade (in French patent 2 782 467) to mount the nut on a frame using acardan joint which allows it to follow the tilting movements of thescrew, to which the cable or another connection component is attached,but the cardan joint must surround the nut in such a way that its axesof rotation converge towards the centre of the nut, which gives toovoluminous an assembly for many applications. Another cardan joint ismoreover provided between the screw and the connection component towhich it is attached.

[0006] A different means is therefore used to improve the alignmentbetween the transmission component such as the cable, the screw to whichthis component is attached and the nut, or at least to attenuate theeffects of alignment defects.

[0007] In its most general form, the invention thus relates to atransmission consisting of a longilinear component, a motor, and a screwand nut assembly, where the nut allows the screw to slide, with the nutbeing driven by the motor and the component being attached to the screw,so as to control the movement of the component by means of the screw viaa conversion of movement between a translation of the nut and a rotationof the screw and where a combination of the component and a means ofattaching the component with arms is flexible in an angular positionrelative to the screw; this transmission is characterised in that thescrew is bored with an axial cavity and that the attachment means ishoused in the cavity.

[0008] The attachment means is, preferably, at mid-travel position ofthe nut on the screw. The result of this design is that the bendingmoment exerted on the screw between the nut and the transmissioncomponent attachment point is proportionate to a short lever arm: it istherefore small.

[0009] The cavity and the longilinear component may emerge from thescrew through one end or through both, particularly if the component iswound in a loop. The component may be a fully flexible cable, a rigidrod or again a flexible rush. The attachment means may then be ratherdifferent. In the case of a flexible part, one setting in the screw willbe sufficient. Otherwise a single or dual articulation, or aball-and-socket joint, will have to be designed in order to allow avariation in the angle between the rod and the screw.

[0010] If the transmission component is not rigid under torsion, a meansof stopping the pivot rotation of the screw relative to a frame or toanother fixed point should normally be provided. This may be atravelling runner pointing in the direction of the screw, or moregenerally a transverse two-degrees-of-freedom coupling like an Oldhamcoupling, or any other coupling having the same property.

[0011] The longilinear component attachment means may be held in thescrew by a spacer engaged in the cavity, against which the attachmentmeans is stopped, and a hollowed-out cap mounted on the screw stoppingon the spacer, and which the longilinear transmission component passesthrough.

[0012] The invention will now be described in more detail by means ofFIGS. 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 which all show differentembodiments of the invention.

[0013]FIG. 1 shows an endless screw 1 passed right through by an axialcavity 2 and in the centre of which is found a setting 3 of atransmission cable 4 which also passes right through the cavity 2 and towhich is fixed a drive part not shown. The cable 4 may form an endlessloop, particularly in this construction, or a segment. The setting 3 isheld, on each side of the cavity 2, by a spacer 5 in the form of a bushwhich extends between it and one of the apertures of the cavity 2, whereit is held by a cap 6 including a nut portion 7 engaged on the lastthreads of the screw 1, a central aperture 8 for the cable 4 to passthrough and a stop part 9 which holds the spacer 5 in place in thecavity 2.

[0014]FIG. 2 shows a similar construction, but one in which the cavity10 is blind and the setting 3 is supported against the bottom of thiscavity. It is then one end of a cable 11 of finite length that is set. Asingle spacer 5 and cap 6 system is encountered.

[0015] The invention may be implemented (FIG. 3) with a rigid rod 12replacing a flexible cable. The setting 3 is then replaced by anarticulation, and in particular by a dual articulation or aball-and-socket joint 13 in which one end of the rod 12 is held. Theother aspects of the constructions in FIGS. 1 and 2 remain possible, andfurthermore a system with a bush acting as a spacer 5 and a cap 6 actingas a stop is encountered again.

[0016] A complete transmission is shown in FIG. 4. A frame 14 houses ahollow shaft motor 15 which drives a hub 16 extending out of the motor15 to form a receptacle 17 for housing a nut 18 driving a screw inaccordance with any one of the previous definitions, like the screw 1 inFIG. 1. IL is engaged in the nut 18 and, depending on the motion that isimposed on it, may penetrate into the hub 16, which is passed through inany case by one branch of the cable 4. The other end of the screw 1carries a pin 19 supporting rollers 20 rolling in wide-open grooves 21of a sleeve 22 belonging to the frame 14. 6 This travelling runnersystem integral with the screw 1 pits itself against rotations of thescrew 1 and therefore allows the nut 18 to slide it along the grooves 21approximately parallel to the screw 1, while lending itself to smalltransverse tilting movements of the screw 1 in the fixed sleeve 22 as afunction of the traction exerted on the cable 4, thanks to the track andaperture widths of the grooves 21. Finally, the hub 16 is held in theframe 14 by a pair of bearings 23.

[0017] The mechanical connection between the screw 1 and the componentguiding it, in this case the sleeve 22, must be approximately equivalentto an Oldham coupling, which has the property of absorbing motion inboth transverse directions of both the parts it joins together. In theprevious embodiment, the rollers 20 may be displaced to one side withoutleaving the tracks of the grooves 21, and in the other direction bylifting themselves from the tracks. It is not necessary for the rollers20 to be closely guided in the grooves 21 since they are used above allto pit themselves against the rotation of the screw 1.

[0018] If the rollers 20 are however arranged in grooves 21 which aretoo narrowly open to allow sufficient travel of the screw 1, thenecessary flexibility may be restored if the rollers 20 are mounted on aframe 30 equipped with a central slider 31, oriented in the direction ofopening of the grooves 21, on which an end piece 32 of the screw 1slides (FIG. 7), or if a leaf support 33, flexible in the samedirection, joins the end piece 32 to a runner 34 carrying rollers 20(FIG. 8). A bellows could replace the leaf spring 33; the assembly wouldthen be flexible in the width direction of the grooves 21, which wouldallow them to be replaced and a rigorous guide bush to be used, takingthe appearance of sliders 35 parallel to the screw 1 and on which wouldslide rings 36 connected to a runner 37, which would also be connectedto an end piece 38 of the screw 1 either by a bellows 39 (FIG. 9), or byan Oldham coupling 40 (FIG. 10).

[0019] It may be seen that the setting 3, located approximately half-wayalong the screw 1 and therefore approximately at mid-travel, remains ata short distance from the nut 18 even when the screw 1 is in its extremeposition: if vertical bending is imposed on it by the cable 4, the forcewhich is exerted between it and the nut 18 remains moderate, whichsubstantially reduces the risks of sticking and bending of the screw 1and of the friction which might be exerted between it and the nut 18even without permanent damage. Experiments conducted in the laboratoryhave demonstrated this, and in particular that the friction measuredremained close to the theoretical values of the screw and nut system.One effect of the invention is therefore to make this system effectiveonce again.

[0020]FIG. 5 is a similar design, in which the hollow shaft motor 15 isreplaced by a motor 24 external to the frame 14 and which drives one endof the hub 16 using a belt drive 25 tensioned between pulleys or thelike.

[0021]FIG. 6 shows a composite transmission component including, withinthe screw 1, a single portion 26 as a single cable, which is joined toan additional portion 27 of the component which is divided at that pointand, composed of a plurality of strands of cables 28 joined to thesingle portion 26 by a spreader 29. This arrangement may be found onboth sides of the screw 1 if necessary. The component may as previouslyform a loop or not form one.

[0022] It is apparent that the invention may be implemented in otherways. In this way the rotation stop component becomes unnecessary whenthe transmission component is rigid under torsion, unable to pivot andconnected to the screw by a connection which is rigid under torsion: inthese constructions, stopping the rotation of the screw 80 is broughtabout by the essential components of the mechanism themselves; in thecase of FIG. 3, the bar 12 is rigid under torsion and therefore holdsthe screw if it is attached to a body which does not rotate and if adual articulation replaces the ball-and-socket joint 13.

[0023] Numerous applications may be proposed for the invention. Thetransmission component may rotate a body, translate it, or even cause itto pivot or otherwise move. It is already planned to use the inventionto regulate the steering of guide wheels, or to act as active suspensionto a wheel suspended from a lever made to rotate by the invention, as acomplement to conventional cushioning means.

[0024] When the transmission component is flexible and has to betensioned, this may be achieved by a spring, a counterweight or a looparrangement of the component.

[0025] It is obvious that the previous description has not exhausted allthe forms the invention may take, and in particular all its possiblemodes of construction; those which have been discussed could be combinedin different ways.

1. A transmission, consisting of a longilinear component (4, 11, 12), amotor (15, 24), and a screw (1) and nut (18) assembly, where the nutallows the screw to slide, with the nut being driven by the motor andthe component being attached to the screw in such a way as to controlthe movement of the component by means of the screw and via a conversionof movement between a rotation of the nut and a translation of thescrew, a combination of the component and of a means of attaching thecomponent to the screw being flexible in an angular position relative tothe screw, characterised in that the screw is bored with an axial cavity(2) and that the attachment means (3, 13) is housed in the cavity.
 2. Atransmission according to claim 1, characterised in that the attachmentmeans (3, 13) is at mid-travel position of the nut on the screw.
 3. Atransmission according to any one of claims 1 or 2, characterised inthat the cavity and the component pass completely through the screw. 4.A transmission according to any one of claims 1 to 3, characterised inthat the component is a flexible cable (4) and the attachment means (3)is a setting.
 5. A transmission according to any one of claims 1 to 3,characterised in that the component is a rigid rod (12) and theattachment means includes a dual articulation.
 6. A transmissionaccording to any one of claims 1 to 5, characterised in that thecomponent is rigid under torsion and the screw is free in rotation.
 7. Atransmission according to any one of claims 1 to 5, characterised inthat the screw is provided with a rotation stop means (19, 20) relativeto a frame (14, 22).
 8. A transmission according to claim 7,characterised in that the rotation stop means includes a travellingrunner pointing in the direction of the screw.
 9. A transmissionaccording to any one of claims 7 or 8, characterised in that therotation stop means includes a two-degrees-of-freedom coupling for thetransverse tilting movement of the screw between the screw and theframe.
 10. A transmission according to claim 8, characterised in thatthe coupling includes wide and wide-open grooves (21) in which rollers(20) of the runner may move transverse to the screw in both directions,the runner (19) being connected to the screw.
 11. A transmissionaccording to claim 8, characterised in that the coupling includes widegrooves in which rollers (20) of the runner may move transverse to thescrew in one direction, the runner being connected to the screw by ameans (31, 33) providing flexibility in one opening direction of thegrooves.
 12. A transmission according to claim 11, characterised in thatthe means is a leaf spring (35).
 13. A transmission according to claim7, characterised in that the rotation stop means includes sliders (35)on which slides a runner (37) joined to the screw by a coupling (39, 40)flexible in two directions transverse to the screw (1).
 14. Atransmission according to any one of the claims 1 to 13, characterisedin that it includes a spacer (9) engaged in the cavity (2), againstwhich the attachment means (3, 13) stops and a hollowed-out cap (16),mounted on the screw (1) stopped on the spacer, and which thelongilinear component passes through.
 15. A transmission according toany one of the claims 1 to 14, characterised in that the longilinearcomponent includes a unitary portion (26) in the screw (1) and a dividedportion (27) outside the screw, which is coupled to the unitary portionby a spreader (29).